double v = (double)new System.Xml.XmlDocument().CreateNavigator().Evaluate(System.Xml.XPath.XPathExpression.Compile(stringVar.Replace("/", "div ")));

        private void button3_Click(object sender, EventArgs e)
        {
                 string error;
                string code = "using System; " +
                "public class MyCalculator " +
                "{" +
                   " public object Calc()" +
                   " {" +
                       " return " + textBox6.Text + ";" +
                   " }" +
               " }";

            object res = MemLibParser.Exec(code, "Calc", null, out error);
            if (res != null)
                textBox7.Text = res.ToString();
            if (error != string.Empty)
                MessageBox.Show(error);

        }


    public class MemLibParser
    {
        private static string[] GetAppReferences()
        {
            List<string> references = new List<string>();
            foreach (System.Reflection.AssemblyName assemblyName in Assembly.GetExecutingAssembly().GetReferencedAssemblies())
            {
                references.Add(Assembly.Load(assemblyName.FullName).Location);
            }
            references.Add(Assembly.GetExecutingAssembly().Location);
            return references.ToArray();
        }

        private static Assembly Compile(string src, out string error)
        {
            error = string.Empty;
            Microsoft.CSharp.CSharpCodeProvider comp = new Microsoft.CSharp.CSharpCodeProvider();

            System.CodeDom.Compiler.CompilerParameters pars = new System.CodeDom.Compiler.CompilerParameters(GetAppReferences());

            pars.GenerateInMemory = true;

            System.CodeDom.Compiler.CompilerResults result = comp.CompileAssemblyFromSource(pars, src);

            if (result.Errors.HasErrors)
            {
                foreach (System.CodeDom.Compiler.CompilerError er in result.Errors)
                {
                    error += string.Format("{0}:{1} {2}", er.Line, er.Column, er.ErrorText) + Environment.NewLine;
                }

                return null;
            }
            return result.CompiledAssembly;
        }

        public static object Exec(string src, string method, object[] parameters, out string error)
        {
            error = string.Empty;
            try
            {
                Assembly asm = MemLibParser.Compile(src, out error);
                Type type = asm.GetExportedTypes()[0];
                object obj = type.GetConstructor(Type.EmptyTypes).Invoke(new object[] { });
                return type.GetMethod(method).Invoke(obj, parameters);
            }
            catch (Exception ex)
            {
                error = error + Environment.NewLine + ex.Message;
                return null;
            }
        }
    }